Process for producing dispersion strengthened nickel with aluminum



Aug. 26, 1969 J w, WEETQN ETAL 3,463,579

PROCESS FOR PRODUCING DISPERSION STRENGTHENE NICKEL WITH ALUMINUM FiledJuly 24. 1967 qv s Mud? ATTORNEYS United States Patent U.S. Cl. 148-1266 Claims ABSTRACT OF THE DISCLOSURE Dispersion strengthened materialscontaining ultra-fine dispersoids from mechanically produced blends ofmatrix and dispersoid powders. Microstructural stability is achieved bycarefully controlling the cleaning and densification of partiallyconsolidated thin shapes.

ORIGIN OF THE INVENTION The invention described herein was made byemployees of the United States Government and may be manufactured andused by or for the Government for governmental purposes without thepayment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION This invention is concerned with improvedmaterials comprising metallic matrices in which are imbedded oxides orother compounds in a very fine form. The invention is particularlydirected to a process for producing such materials in which the matrixis cleaned without appreciably agglomerating the added dispersoid.

Metals and alloys with fine dispersoids have high strength properties atelevated temperatures close to the melting point of the matrix material.These properties increase as the particle size and spacing decrease. Thestarting particle size should be less than one micron.

Submicron particles of both metal and oxide are commercially availableor can be obtained by adding materials to a grinding media that tend toreduce fine particles close to the colloidal state. In using ultra-finepowders of metals with large surface areas problems of achieving highpurity in a final product are encountered.

The use of a reduction-type cleaning process has been utilized in anattempt to solve these problems. In such a process, matrix metal powderis cleaned in hydrogen, before or after blending with an oxide, atrelatively low temperatures to avoid sintering. The cleaned powder isthen consolidated into a billet by hot pressing or cold pressing andsintering.

The efliciency and completeness of this process is limited because incleaning fine powders, some powder may be sintered at the cleaningtemperature and trap reaction products such as water vapor andcarbonaceous gasses. The cleaning gas may not reach all the powder atthe interior when cleaning cold compacted billets or deep beds ofpowder, and reaction products may recombine with clean material beforereaching the surface. In cleaning partially densified billets or largespecimens, the surface may sinter so that the cleaning gases cannotenter or the reaction products escape. Even wellcleaned materials may berecontaminated during subsequent processing before they are completelydensified in the final product. As a result of these problems anddifiiculties encountered in cleaning, fine powder dispersionstrengthened materials produced by mechanical methods have exhibitedmicrostructural instabilities.

These problems have been solved by the present in- 3,463,679 PatentedAug. 26, 1969 ice vention in which a compact of partialy densifiedpowder has a cross section sufficiently thin to permit easy access ofthe cleaning gases. These powders are partially densified intoconfigurations in which one or more dimensions are limited so that thecleaning gas readily moves to the interior of the compact and thereaction products easily escape. Also, a slow heating rate is utilizedto permit elimination of the impurities prior to final and completedensification. Careful control of cleaning and densification rates isutilized, and the clean dense product is not readily recontaminated.

OBJECTS OF THE INVENTION It is, therefore, an object of the presentinvention to provide a stable dispersion strengthened product by using acompact having a thin cross section so that all of the material isreadily accessible to cleaning gases.

Another object of the invention is to provide a stable, mechanicallyblended, dispersion strengthened material having an ultra-finedispersoid of a highly stable oxide by careful control of cleaning anddensification rates.

A still further obect of the invention is to provide dispersionstrengthened materials containing ultra-fine dispersoids by mechanicallyblending powders followed by consolidation to a suitable density andcareful cleaning so that the resulting microstructures are stable.

These and other obects of the invention will be apparent from thespecification which follows and from the drawing wherein like numeralsare used throughout to identify like parts.

DESCRIPTION OF THE DRAWING The drawing is a perspective view showingcoins made from powder blends stacked in a rack for cleaning inaccordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Powder blends are wet ballmilled, washed, and pressed into compacts in the form of coins 10 shown'in the drawing. The coins 10 are placed on discs 12 in a rack 14 andheated in hydrogen at a programmed rate. The cleaned and sintered coins10 are then annealed in hydrogen.

To illustrate the features of the invention nickel powder having aparticle size of 2.5 microns and five volume percent of 0.03 micronalumina were ground for 72 hours in heptane under an argon blanket. Thegrinding balls, stirrers, and mill container were all made of nickel toavoid contamination. As the grinding proceeded, gradual additions ofoleic acid were made in the blend so that the total at the end of thegrinding was 20 percent by weight of powder. The ground material wasthen washed ten times with heptane and two times with acetone bydecantation in an air atmosphere using a centrifuge to accelerate thesettling of the powder. The average particle size of the ground materialwas 0.02 to 0.05 DJICIOI].

One half of the slurry was dried in air, and the powder transferred to acleaning tube. The powder was first maintained at a reduced pressure ofabout 25 torr and 600 F. for one day and then cooled to roomtemperature. The temperature was then raised slowly to 1000 F. whilemaintaining a flow of 1-0 cubic feet of hydrogen per hour through thepowder. The hydrogen was purified in a palladium diffuser, and themoisture level of the effluent hydrogen was monitored. Increases oftemperature were controlled so that the moisture evolution wasmaintained below 1000 p.p.m. The temperature and the flow of hydrogenwas maintained for about three days until the moisture reading in theeflluent gas fell below 10 p.p.m.

The powder was pressed into two inch diameter coins shown in thedrawing. The pressing was done in air using a 20,000 pound load, andeach coin 10 had a thickness of inch. The coins 10 were then placed onporous discs 12 mounted on the rack 14. All the coins It) on the rack 14were simultaneously processed through the cleaning and densificationsteps.

The rack 14 comprises arms 16, 18, and mounted on a circular base 22. Acircular top plate 24 is mounted at the opposite end of the arms fromthe base 22. The arms 16, 18, and 20 extend vertically between the base22 and the top plate 24. Both the base 22 and the top plate 24 contain aplurality of holes through which cleaning gases pass.

A plurality of supports 26 are fastened on each of the arms. Eachsupport 26 has a long leg 28 which engages a lower surface of one of thediscs 12 and a short leg 30 which engages the outer peripheral surfaceof an adjacent disc 12. The discs 12 are of zirconia and are porouswhich facilitates the passage of cleaning gases to the coins 10. In thismanner, each of the coins It) has all of its surfaces exposed to thesurrounding atmosphere to promote entrance and exit of gases.

The entire rack 14 was placed in a hydrogen tower, and the coins 10 wererecleaned using the same schedule as for the powder. The rack 14 and thecoins 10 were then sealed in plastic under argon and transferred to asintering furnace. After removing the plastic under an argon blanket,the coins 10 were slowly heated to 2000 F. in hydrogen at a rate of lessthan 200 F. per hour. A two hour hold at 800 F. and a two hour hold at2000 F. in hydrogen was followed by six hours in vacuum at 2000 F.

per million. The rate of heating was controlled by either the outgassingor moisture evolution and did not exceed 200 F. per hour.

Small pellets were also prepared by cartridge-actuated compactionpressing. These pellets had a diameter of inch and a thickness of ,4,inch. Each pellet was made at room temperature by explosive compactionin a specially constructed die using a .45 caliber cartridge. Thepellets were annealed for one hour at 1600 F. in a vacuum of 10 torr toeliminate the worked background and improve the detail of the oxideparticles in electron micrographs. The powder used for the pellets wascleaned in hydrogen at 800 F. prior to compaction. The particle sizesand interparticle spacings of the compacted and annealed pellets wereclose to that of the initial blend.

The microstructural parameters of blended, cleaned, and annealednickel-alumina coins are shown in the table. The standard stabilityanneals of the cleaned and densified coins were conducted-in hydrogen.These anneals were 24 hours at 2300 F. and 11 hours at 2300 F. followedby 11 hours at 2600 F. as set forth in the left column of the table.

The parameters, such as volume percent of particles, interparticlespacing, and particle size listed in the table were determined by usingmeasurements taken from electron micrographs. Representative electronmicrographs were selected and area analyses were made of them. Aparticle size analyzer was used to determine the numbers of particles ofgiven diameter in each electron micrograph.

Referring again to the table, the measured interparticle InterparticlePartrcleslze, Volume, spacing, Specimen condition micron (percent)micron Blend:

Cartridge actuated compaction 0. 010-0. 19 3 44 1 04 Stress anneal 1,600F., 1 hr. vacuum 0. 022 \Yet method:

Cleaned through 2,000 F. .f 0. 017-8. gi G7 1 Stability annealed H2 0:512,a00 24n1- 0.13 L14 Stability annealed, H 0-0. 3 31 6 U3 2,300 F., 11hl'., 2,600 0.10 l tl l 2 000 1 0 01" 0 "3 eane 11011" 1 1- .4 a 1;. Hrrata taiiyanneae 2 2,300 F., 24 50%- 0.14 48 Stability annealed, H2 0.086-0. 60 2 F 7 90 2,300 F., 11 1m; 2,600 F., 1111 50%-0. 23

A number of coins 10 ranging in thickness between & inch to 4 inch wereproduced to determine the optimum thickness. The thinnest coins werefound to warp on processing while the thicker coins were not completelycleaned and densified at the center. The 4; inch thick coins retainedtheir shape, were readily and uniformly cleaned and densified, andcontained a good distribution of fine particles of the added oxide.

Wet compacted nickel-alumina coins were also cleaned and densified. Toproduce these coins the remaining half of the powder slurry made in themanner previously described was washed with alcohol followed by water toeliminate the organic solvent. The wet powder was pressed directly intocoins having a diameter of two inches and a thickness of V; inch using a20,000 pound load.

These coins were likewise placed on porous zirconia discs 12, stacked onthe rack 14 and transferred to a sintering furnace. The coins were thenheated slowly and held successively for two hours in a vacuum at 600 F.,two hours in purified hydrogen at 800 F., two hours in hydrogen at 2000F., and six hours in vacuum at 2000 F. The hydrogen was purified with apalladium diffuser, and the moisture content of the hydrogen emittingfrom the purifier was measured to be 2 to 3 parts spacing of theas-blended material was 1.04 microns and the median particle size was0.022 micron. The cleaning of the wet material at a temperature of 2000F. increased the interparticle spacings to 1.8 microns and the medianparticle size to 0.042 micron which is almost double that of the initialsize. A 2300 F. stability anneal increased the median particle size to0.13 micron and the interparticle spacing to 3.4 microns. In this case,there was a relatively large amount of growth of the oxide.

Annealing at 2600 F. resulted in a further growth of the oxide particlesto a median size of 0.19 micron and increased the interparticle spacingto 6.03 microns. However, some oxide particles as small as 0.05 micronwere still present. In all cases, the volume percent oxide measured waslower relative to the nominal quantity added to the ball milled.

The dry coins cleaned at 2000 F. exhibited a larger interparticlespacing than did the wet coins, but the median particle sizes were thesame for both. Annealing at 2300" F. increased the interparticle spacingto 4.67 microns. Again, the median particle size was approximately thesame as for the wet coins. Increasing the annealing temperature to 2600F. resulted in a further increase in both particle size andinterparticle spacing.

The parameters listed in the table show that the pre liminary partialdensification of the coins traps the added oxides in place in thepresence of impurities but permits thorough cleaning to occur beforedensification is complete. A major portion of the cleaning is believedto occur at the low temperature, and. the remaining impurities areeliminated as the temperature is raised in successive steps. Finalcomplete cleaning of the traces of impurities is accomplished at theelevated temperatures.

After the coins 10 have been cleaned in accordance with the invention,larger articles are made by stacking the coins 10 together, sinteringthese coins or isostatically hot pressing them, and subsequently canningand extruding the product. It is also contemplated that similar articlescan be produced by balls or pellets rather than coins. It is importantthat the cross section of the pellets or balls be sufficiently small toenable them to be cleaned in the aforementioned manner.

Other dispersioned strengthened materials that can be made in accordancewith the invention. Nickel powder, 200 to 300 angstroms in size andcontaminated with 0.25 percent by weight sulphur was wet-mixed with 8volume percent thoria. The resulting mixture was dried, the powdercleaned in hydrogen at 100 F. for 80 hours, and then sintered at 1900 F.in hydrogen.

This same mixture was processed into coins 10 shown in the drawing of'both the wet and dry types by the methods previously described inconnection with the nickel-alumina blends. Photomicrographs showed thethoria particles in the coins to be much finer than those in thesintered powder.

Pure thorium oxide powders of an ultra-fine size of 50 to 150 angstromswere blended with nickel by ball milling. The blend was cleaned andannealed at 2000 F. and further annealed at 2300" F. in the mannerpreviously described. The particle sizes range from 0.018 to 0.382micron with the median being 0.059 micron. The interparticle spacing was2.26 microns.

A fine grind of a chromium bearing nickel alloy having a composition of55Ni-20Cr-25W without stable oxide particles added to the material wascompacted by a cartridge-actuated compaction press. This material hadlarge quantities of impurity oxides in the consolidated powder. Thenthis same ground chromium bearing material was compacted in thepreviously described manner, and cleaned in accordance with theinvention at a temperature as high as 2300 F. in hydrogen.Photomicrographs showed that the majority of the oxide was removed. Whenrepeated with the addition of submicron thoria a fine distribution ofthe added oxide was retained.

While several embodiments of the invention have been described, it willbe appreciated that various changes and modifications can be madeWithout departing from the spirit of the invention or the scope of thesubjoined claims.

What is claimed is:

1. A method of making a dispersion strengthened material from powders ofa nickel matrix and an alumina dispersoid comprising the steps of:

mixing the nickel matrix powders with the alumina dispersoid powders,

pressing the mixed powders into a plurality of coins,

each having a thickness between inch and A inch,

stacking said coins in spaced relationship so that each coin has all ofits surfaces exposed to the surrounding atmosphere,

heating said spaced coins in a vacuum to stress anneal the same,

passing hydrogen through said spaced coins to clean the same, monitoringthe moisture level of the eflluent hydrogen, heating said coins to 2000F. at a rate that maintains the moisture evolution below 1000 parts permillion, interrupting said hydrogen flow and said heating when saidmoisture content of the effiuent hydrogen is below 10 parts per million,and

heating said cleaned coins in hydrogen to at least 2300" F. for at least11 hours to stability anneal the same.

2. A method of making a dispersion strengthened material as claimed inclaim 1 including the step of heating the cleaned coins in hydrogen toat least 2300 F. for 24 hours to stability anneal the same.

3. A method of making a dispersion strengthened material as claimed inclaim 1 including the step of heating the cleaned coins in hydrogen to2300 F. for ll'hours and then heating said coins to 2600 F. for 11 hoursto stability anneal the same.

4. A method of making a dispersion strengthened material as claimed inclaim 1 including pressing the powders into coins having a thickness ofabout inch.

5. A method of making a dispersion strengthened article as claimed inclaim 1 including the step of working a plurality of consolidated andcleaned coins into a predetermined configuration.

6. A method of making a dispersion strengthened material as claimed inclaim 1 wherein the mixed powders are dried prior to pressing.

References Cited UNITED STATES PATENTS 1,377,982 5/1921 Keyes 224 X2,598,796 6/1952 Hulthen 75224 X 2,793,116 5/1957 Cuthbert 752242,806,786 9/1957 Kelley 75224 X 2,860,972 11/1958 Fraser 75---224- X3,063,836 11/1962 Storchheim 75--224 3,070,440 12/1962 Grant 752063,218,697 7/1962 Wainer 75206 X 3,310,400 3/1967 Alexander 752063,357,826 12/1967 Honaker 1. 75206 3,366,479 1/1968 Storchheim 75-206 XCARL D. QUARFORTH, Primary Examiner ARTHUR J. STEINER, AssistantExaminer US. Cl. X.R. 75206, 211, 224

